Histomoniasis (Blackhead disease) in Game Birds

This usually acts with secondary bacteria to cause lesions in
the liver which can be visualised in post mortem examinations. Only
in very exceptional circumstances does the disease cause a
black/purple colouring of the head of the bird. This name comes
from historic cases in turkeys which literally get a 'black head'.
Histomoniasis affects turkeys to a greater extent than chickens or
game birds. The caecal worm (Heterakis gallinarum) eggs
and larvae can carry the parasite and the larvae can also
themselves be carried by earthworms and both sources when eaten by
the bird can cause worm infection and subsequent blackhead. Game
birds can also pick up the Histomonas parasites directly from the
faeces of infected birds, but in most cases it is the caecal worm
that is the culprit. The Blackhead parasite can live in the caecal
worms for up to four years, and the worm acts as a means of
protection for the parasite from the environment. As earthworms can
also be carriers of Blackhead, land can remain infected with the
parasite for a considerable length of time.

Adult heterakis worms in the caecum of a pheasant

CLINICAL SIGNS

Clinical signs of Histomoniasis can include a dull and listless
temperament in the bird, a frothy or bright yellow diarrhoea,
weight loss and sudden death. Very rarely there is a black/purple
colouring of the head.

POST MORTEM FINDINGS

Post-mortem examination shows enlarged caecae and typical
lesions, usually in the so-called 'nail head' or 'horses hoof'
pattern, on the liver of the bird. It is this liver damage that
typically causes mortality. Often ulcers can be seen in the caecum
wall - where the parasite lives before it burrows into the
liver.

Typical blackhead lesions in the liver

DIAGNOSIS

Blackhead disease is diagnosed via a post mortem examination and
the finding of the typical lesions in the liver and caecae.
Definitive diagnosis can also be achieved by histopathology where
portions of the affected blind gut or liver are examined under the
microscope to look for the parasites.

TREATMENT AND CONTROL

Emtryl was an effective treatment of Blackhead, but has now been
banned. Since then, all treatments are aimed at preventing
secondary bacterial infections. It is essential that birds infected
with Blackhead are wormed to kill the Heterakis worms that may be
carrying the parasite, and this should be repeated again after 14
days. Oregano essential oil containing compounds may have some
effect against the parasite and help to reduce the chances of
blackhead infection.

Some of the best Blackhead disease prevention methods are good
management practices. Good control of worms through regular
worming, keeping away wild birds and having good biosecurity can
all be effective control methods - as is a fresh foot dip and clean
wellies!! Although the oral route of infection is considered the
most likely, it has been shown in turkeys and chickens that the
parasite can enter via the vent when the birds are kept on dirty
litter. There is no reason to suggest this cannot also occur in
other species, so good hygiene plays its part in controlling the
disease. Preventing the birds having access to feed spilled on the
ground together with giving the birds access to clean, dry areas is
also important.

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